The production of chemical wood pulp is divided in two areas, i.e. the fibre line area where the chemical pulp is produced with waste liquor as a byproduct and the chemical recovery area, where chemicals needed in the fibre line area is recovered from the waste liquor. The single most expensive piece of equipment in the whole pulp mill is the chemical recovery boiler and the total pulp production capacity is heavily dependent on the capacity and availability of the recovery boiler. If the recovery boiler becomes the bottleneck in the mill, it will have a direct impact on the ability of the mill to increase the pulp production capacity.
The recovery boiler reaches its capacity limitations when heating surfaces are plugged because of "carry-over" of physical particles from the lower part of the combustion chamber. The amount of carry-over depends on four parameters; the upward gas velocity, the particle (droplet) size, particle density, and the number of particles with unfavorable properties. When the capacity increases more air is needed, and this will increase the upward gas velocity. Higher capacity also increases the number of unfavorable particles and the combined effect is that carry-over increases with increasing load. The boiler has reached its capacity limit when the boiler becomes plugged because of carry-over and the mill is unable to produce more pulp unless a huge investment is done in a new recovery boiler or a costly retrofit of the existing boiler, which also requires a long outage for installation and additional production losses and loss of revenue as a consequence.
Already in 1982 it was shown practically, e.g. by the paper "Tillsats av syrgas vid forbranning av sulfittjocklut vid MoDomsjosulfitfabrik, S. Larsson, AGA, C. Nilsson, MoDo, L. Saltin, AGA, Svenska Sodahuskonferensen, Stockholm, Sweden, Nov. 18, 1982" and by the brochure "Oxygen Enrichment increases Capacity, AGA AB, GM164e (1983)", that by enriching primary and secondary combustion air with oxygen enriched air up to 23% (by volume) oxygen content, the capacity of a sodium sulfite recovery boiler could be raised significantly.
U.S. Pat. No. 4,857,282, disclosed in 1988 a way to process black liquor by enriching the primary and/or secondary normal process air levels used in the combustion process by addition of pure oxygen in the amount of 0.63 kg/kg ds and by addition of 0.42 kg of oxygen from air/kg ds (dry solids), for the combustion of one kg of incremental dry solids, which means a total oxygen content of 21.8% by volume if the additional oxygen is evenly distributed to the air streams or in the extreme case if all the additional oxygen is added to only one of the two air streams up to an effective amount of 5% oxygen by volume to said air stream the absolute oxygen content of said airstream will raise to 24.8% if the split between primary air/secondary air is reduced to 23/77% (of volume) of total air supplied. This patent states that the incineration rate or capacity of the recovery boiler can be increased by a moderate supply of oxygen to the primary and/or secondary air stream in three ways; 1. An increase in the adiabatic flame temperature which will increase the heat flux in the lower furnace and 2. An increase in the char burning rate since the char burning rate is a linear function of oxygen concentration, and 3. Increase in the drying rate by an increased lower furnace temperature.
This is basically a subset of the results from a recovery boiler experienced 6 years earlier and reported in the two first mentioned publications.
The drawback with both these known methods is that by enriching combustion air with oxygen enriched air in the lower furnace, i.e. primary and secondary air registers without reducing the air factor (the air factor is defined as the actual oxygen supply divided by the stoichiometric supply of oxygen for complete combustion) the conditions for NOx formation will increase because of higher temperature and increased volume where there is oxidizing conditions in the lower furnace. NOx emission will now be the limiting capacity factor due to the strict environmental regulations. The first two publications describe some theoretical calculations showing that by redistributing the oxygen between the air registers, the temperature can be controlled. The flexibility of these boilers from the 80's was very limited due to air registers located in the lower furnace underneath the liquor sprayers and the air factor was normally above 1 at the liquor gun elevation (i.e. stoichiometric or above) regardless how the oxygen was redistributed between the registers.
Today the recovery boilers are using "plain" air to achieve a total air factor of 1-1.05 entering the superheater section and substoichiometric conditions in the lower furnace by the addition of air levels in the upper furnace, so called overfire air registers or tertiary, quarternary etc. air registers. This is schematically illustrated in the accompanying FIG. 1. Today the common practice is to redistribute combustion "plain" air from the lower furnace to the upper furnace in order to maintain a NOx level within regulatory limitations. This can be done because the quality of the liquor as a fuel has improved. The liquor dryness has increased substantially in the past 10 years, which means that the "as fired" heating value has gone up, which facilitates the redistribution of "plain" combustion air to this new upper furnace air levels.
The purpose of the present invention is to provide a method combining in a new manner the positive effects achieved with the oxygen enriched air in accordance with the above two first mentioned publications with the advantages of the modern recovery boiler design of today in order to further reduce the air factor in the lower furnace, to maximize the capacity and to minimize emissions, and this has been achieved in accordance with the contents of the attached claim 1.